There are many occasions that one wraps a towel or blanket around their shoulders. At home, often after being comfortable under a blanket, one wraps the blanket around their shoulders when getting up to keep warm. Likewise, after a shower, one often wraps a towel around their body to keep warm and continue drying.
At the beach or pool, typically after bathing, one may wrap a towel around their upper torso to dry off as well as keep warm, especially when there is a breeze.
When standing relatively still and not performing other activities, it is often easy to keep a hold on the blanket or towel to keep it from falling off. There are many times when this is not the case. It is not easy to keep the towel or blanket on one's shoulders when performing other operations such as picking up beach paraphernalia, cooking, tending to children, etc. Wrapping of a towel around one's waist is more secure than covering one's shoulders, but wrapping a towel around one's waist does not dry and keep the upper torso warm.
What is needed is a towel/blanket that will stay in place around one's shoulders without occupying the wearer's hands, thereby freeing the wearer's hands to perform other activities.